The present application relates to the magnetic resonance art. It finds particular application in conjunction with cylindrical bore magnetic resonance imagers with annular magnets and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other types of magnetic resonance imagers.
Heretofore, magnetic resonance imaging systems have commonly included a series of annular superconducting magnets surrounded by a toroidal vacuum dewar. Gradient magnetic field coils were disposed circumferentially around a central bore of the vacuum dewar, often supported on a dielectric cylindrical former. A radio frequency coil was commonly disposed inside of the gradient coils, often on another cylindrical former.
In order to maximize the size of the central patient receiving bore, hence the size of the patient which could be received, it was desirable to mount the radio frequency coil on as large a diameter cylinder as possible. On the other hand, the cost of the superconducting main field magnets increased with size and diameter. Hence, it was desirable to make the main field magnets as small a diameter as possible. In magnetic resonance imaging, the gradient field coils were called upon to create magnetic field gradient pulses. The sharpness or crispness of the magnetic field gradient pulses was a limiting factor on the resolution and sharpness of the resultant image. The smaller the size, i.e., the smaller the diameter, of the gradient field coils, the sharper and more crisp the gradient field pulses tended to be due to increased efficiency and slew rate and reduced eddy current effects from interaction with cold shields. In order to accommodate these competing interests, the radio frequency and gradient magnetic field coils were typically mounted close together such that both had nearly the same diameter.
The gradient coils are pulsed by applying pulses of high amperage kilohertz frequency current. The high power pulses applied to the gradient coils created electrical noise which corrupted the signal received by the radio frequency coil. To protect the radio frequency coil from this noise, a metallic shield was typically located between the gradient coil and the radio frequency coil. In general, the thicker the layer of metal, the more effective the shielding. Often, the metallic shield was positioned either on an inside diameter of the dielectric former which carried the gradient field coils or was laminated to the exterior diameter of the dielectric former before the gradient coils were applied. See, for example, parent U.S. patent application Ser. No. 08/080,413 of Morich, DeMeester, Patrick, and Zou.
In order to meet clinically driven demands for faster higher resolution imaging, the gradient strength and slew rate have been increasing, i.e., the gradients are becoming larger and crisper. One difficulty encountered was that the large gradient pulses induced corresponding eddy currents in surrounding metal structures, such as the metallic shield. The eddy currents limited the slew rates.
One approach for limiting eddy currents was to cut the solid copper sheet into smaller elements to reduce the size of the material available to conduct eddy currents, hence reduce eddy currents. See, for example, U.S. Pat. No. 4,642,659 of Hayes and U.S. Pat. No. 5,243,286 of Rezedzian.
One approach for limiting eddy currents was to replace the solid copper sheets with a copper mesh. Because the mesh typically had less metal that the metal sheets, shielding was compromised. Another approach was to replace the solid copper sheets with strips of copper tape coated with an insulator. The tape extended longitudinally along the cylinder and adjacent tape strips were lapped to create a capacitive coupling. The effective capacitive coupling between the lapped copper tape strips provided an effective short circuit for radio frequency currents, e.g., in the megahertz range. However, the capacitive coupling appeared as an open circuit to low frequency components. One of the disadvantages of the lapped copper tape was that the low impedance path for RF currents was enhanced by very thin dielectric coatings on the strips. First, it was relatively difficult to obtain sufficiently thin dielectric coatings on the tape or strips. Second, thinning of the dielectric coating promoted arcing.
A further difficulty with the laminated tape strips and the segmented copper sheet was that they provided an effective closed path only for the radio frequency eddy current components. Kilohertz frequency induced currents tended to arc between the strips. The arcing caused noise spikes which again degraded the resultant images.
Although the use of copper strips and copper mesh limited eddy currents, the eddy current problems remained only on a reduced scale. As gradient strength and slew rates have continued to increase, it has been found that the copper strips and copper mesh still support deleterious eddy currents.
The present invention contemplates a new and improved radio frequency shield which overcomes the above-referenced problems and others.